While many cyclic phosphorus-containing chemicals have been proposed and used as stabilizers for a variety of organic materials subject to degradation by heat, oxygen, ultra-violet light, etc., many of them are limited to specific applications, and or have certain deficiencies. For example, tris(.beta.-naphthyl)phosphite has been tested as a stabilizer for vinyl chloride polymers, and has been suggested as a stabilizer for polyamides, but this phosphite has been found to be unsatisfactory as a stabilizer for hydrocarbon polymers. Also, some of the more effective cyclic phosphite stabilizers for poly(olefins) such as distearyl pentaerythitol diphosphite are hydrolytically unstable, both on the shelf and in the polymer. Cyclic phosphorus-containing stabilizers that provide improved heat stability to organic materials subject to degradation, that are less susceptible to hydrolysis, and that form improved combinations with other stabilizers are desired.